The present invention relates to an image forming apparatus comprising multiple image carriers on which images with different color components are formed respectively, and driving means for rotating the image carriers, and to an image forming adjustment method, wherein multiple adjustment images are formed at a predetermined interval in an image forming region corresponding to the cycle length of one rotation of each image carrier, the divergence between the predetermined interval and the detection interval of each adjustment image is detected, the reference phases for the rotation of the respective image carriers are determined, and the driving, means is controlled so that the respective reference phases determined are aligned.
For example, an image forming apparatus in which, on black, cyan, magenta and yellow photoconductive drums, images with the respective color components are formed and transferred to a transfer belt so as to be superimposed is used as an apparatus that forms color images on sheets. In this apparatus, the laser beams outputted from multiple laser diodes corresponding to the respective photoconductive drums are reflected using multiple polygon mirrors corresponding to the respective photoconductive drums and applied to the respective photoconductive drums, thereby forming images with color components on the respective photoconductive drums. However, another apparatus is also used in which the laser beams outputted from multiple laser diodes are applied to a common polygon mirror, and the laser beams reflected using the polygon mirror are applied to the photoconductive drums respectively corresponding thereto. With this configuration, the number of the polygon mirrors is reduced.
This kind of image forming apparatus has a problem of causing low image quality owing to the divergences in the positions of the images with color components transferred to the transfer belt. For the purpose of solving this problem, image forming timing adjustment images (hereafter referred to as marks) are formed, the positions of the formed marks are detected, and image forming timing adjustment is carried out on the basis of the detected positions. The marks are formed such that marks for the color components, black, cyan, magenta and yellow, are formed on the transfer belt sequentially, and the mark forming timing is mainly determined by the output timing of the laser beams.
However, separately from the output timing of the laser beams, for example, owing to the fluctuation or the like in the rotation speed of the photoconductive drum caused by the eccentricity or the like of drive gears, there is a problem of causing divergences at the mark forming positions. Noise owing to the fluctuation in the rotation speed of the photoconductive drum has periodicity in many cases. For the purpose of solving this problem, the reference phases, each being used as the reference for one rotation (one cycle) of each photoconductive drum, are aligned (for example, refer to Japanese Patent Application Laid-open No. 2003-177588).
FIG. 1A is a conceptual view showing an example of a divergence between a reference position and a mark position, and FIG. 1B is a conceptual view showing an example of two kinds of mark positions whose reference phases are aligned. In FIGS. 1A and 1B, the reference positions are positions obtained by dividing the image forming region corresponding to the cycle length of one rotation of the photoconductive drum, on the surface of the drum, at equal intervals. Furthermore, although the marks are attempted to be formed at the reference positions, the positions at which the marks are formed are diverged owing to the fluctuation in the rotation speed or the like of the photoconductive drum. As shown in FIG. 1A, some marks are diverged in the movement direction, and others are diverged in the opposite direction. The divergence (being negative in the movement direction) of the mark with respect to the rotation phase of the photoconductive drum is ideally a sine curve. Hence, for example, it is possible that the center portion between the convex portion (positive peak) and the concave portion (negative peak) of the curve of the divergence is set as the reference phase. Because the reference phases of the respective photoconductive drums are aligned, the divergences at the mark formation positions owing to the fluctuation in the rotation speed or the like of the photoconductive drum are caused similarly, and the divergences at the formation positions become less conspicuous as shown in FIG. 1B.